1. Field of the Invention
The present invention relates to an optical device for use in opto-electronic integrated circuits and the like which are needed in fields of optical communication and the like, and particularly, to an optical device with an optical coupler formed in a light waveguide for effecting light wave branching and/or combining and an optical amplifier which is suitable for use in opto-electronic integrated circuits having a plurality of transmitter and/or receiver portions for optical communication and similar devices or systems.
2. Related Background Art
In recent years, there has been proposed a structure in which a V-shaped groove is formed at a crossing part of crisscross waveguides as shown in FIG. 1 to build a coupler 71 for controlling transmission and reflection of light waves and in which the combining and/or branching of the light waves are conducted between a main waveguide 74 and two sub-waveguides 72 and 73 connected to the main waveguide 74.
In FIG. 1, optical amplifier portions 75 and 76 are arranged in the main waveguide 74 that forms a bus line, and the sub-waveguides 72 and 73 are respectively connected to receiver and transmitter portions (not shown) via optical fibers 77 and 78. The optical amplifier portions 75 and 76 respectively are composed of traveling-wave type lasers for directly amplifying a light signal on the main waveguide 75. The optical fibers 77 and 78 are respectively coupled to the sub-waveguides 72 and 73 by using butt coupling. Ratios of the light branching and combining are adjusted by controlling the light electro-magnetic field profile in the waveguide and the groove depth of the coupler 71. The groove can be formed by etching using fine working techniques such as Ga-focused ion beam (FIB) and reactive ion beam etching (RIBE).
Further, in FIG. 1, reference numerals 81 and 82 are optical fibers forming the bus line, and reference numerals 83a-83d are antireflection coats. In the structure of FIG. 1, multiplexed light signals are transmitted from the transmitter portion (not shown) and propagated through the optical fibers 81 and 82 in opposite directions via the optical coupler 71, and parts of light signals propagated through the optical fibers 81 and 82 are branched by the optical coupler 71 to be guided to the receiver portion (not shown) through the sub-waveguide 72 and the optical fiber 77. The light signal thus received by the receiver portion is demultiplexed and detected to generate desired information.
The structure of FIG. 1, however, has the following drawbacks. First of all, high process accuracies such as positional accuracy and depth control accuracy are needed for the coupler 71, and hence its yield and reproducibility are lowered. That is, strict accuracy is needed to the process since the manner of the light combining and branching is determined from the formation of the coupler 71 relative to a field profile of the light wave propagated through the light waveguides 72-74.
Further, in the structure of FIG. 1, while it is possible to perform the coupling of the light wave at equal ratio from the sub-waveguides 72 and 73 (the transmitter and receiver parts are connected thereto) to the main waveguide 74 or bus line in opposite directions, the branching ratio from the main waveguide 74 to the sub-waveguide 72 is different from that from the main waveguide 74 to the sub-waveguide 73. That is, compared with the branching ratio (e.g., -3 dB) toward a lower portion (i.e., a closed side) of the V-shaped coupler 71, the branching ratio (e.g., -6 dB) toward an upper portion (an open side) of the V-shaped coupler 71 is small. As a result, it becomes impossible to branch the light wave propagated through the main waveguide 74 into light waves propagated through the sub-waveguides 72 and 73 with equal intensity.